How to access Render Result pixels from python script?
unfortunately accessing render results cannot be done via bpy.data.images['Render Result'].

There is a workaround by using Viewer node. basically you will need to attach a Viewer node to your render layer and then access pixels of the Viewer.

see this link: https://ammous88.wordpress.com/2015/01/16/blender-access-render-results-pixels-directly-from-python-2/